Oxy-fuel gas cutting torches are useful for cutting ferrous alloys. With the proper equipment, cuts can be effected through very thick billets. In operation, an oxy-fuel torch is used to direct an ignited stream of oxygen and fuel gas onto the surface of the metal to be cut. The metal is thus heated to its ignition temperature, at which point a stream of cutting oxygen directed at the surface oxidizes the heated metal to effect the cut.
The cutting torch may be one of a premixed or a postmixed type torch. In a premixed torch, preheat oxygen and fuel gas are mixed within the torch head before being discharged for ignition. In a postmixed cutting torch, the preheat oxygen and fuel gas are discharged from the torch in unmixed streams. Turbulence in the discharged streams mixes the oxygen and fuel gas before ignition occurs. A principal advantage of the postmixed cutting torch is that it is not subject to flashback, a potential hazard associated with the use of premixed torches. Flashback occurs when the oxygen and fuel gas mixture in a premixed torch ignites within the torch head. Postmixed torches are therefore preferred for heavy industrial applications where a torch is subjected to considerable heat. A further advantage of the postmixed torch is that postmixed nozzles produce a longer heat zone than premixed nozzles. This permits the postmixed torch to operate farther from the work, decreasing the heat stress on the torch and increasing the service life of the nozzle.
An example of a prior art postmixed oxy-fuel gas cutting torch and nozzle are taught in the U.S. Pat. No. 4,455,176 which issued to Fuhrhop on Jan. 19, 1984. That patent describes a combination cutting torch and nozzle assembly for postmixed oxy-fuel cutting using two separate annular streams of preheat oxygen gas surrounding the fuel gas stream with the inner annular preheat oxygen stream directed to impinge the fuel gas stream very close to the point of discharge from the nozzle assembly. The nozzle assembly is secured to the head of the cutting torch by a hollow retaining nut which forms an annular gap with the nozzle assembly for discharging the outer preheat oxygen gas stream.
All prior art postmixed nozzles for oxy-fuel gas torches operate in substantially the same way. A stream of cutting oxygen is discharged from an axial bore in the nozzle. A plurality of fuel gas discharge orifices arranged in a concentric ring around the axial bore discharge preheat fuel gas and a second plurality of gas discharge orifices arranged in an outer concentric ring discharge preheat oxygen which acts as an envelope that surrounds the fuel gas stream. As the gas streams flow toward the workpiece, a mixing of the fuel gas and the oxygen occurs and the mixture ignites to heat the workpiece.
Testing has shown that up to 50% of the preheat oxygen stream discharged from prior art postmixed torch nozzles is lost to the atmosphere before mixing with the fuel gas occurs. This contributes to inefficient combustion and slows the heating process. It also contributes to the cost of cutting since gases are not utilized to their potential. It has also been observed that prior art postmixed torch nozzles are incapable of effecting a parallel-sided cut through a thick workpiece. The cut is narrower along a top of the workpiece than along a bottom of the workpiece. The thicker the workpiece, the wider the cut at the bottom side. If many thick billets must be cut, a significant loss of metal occurs.
A further disadvantage of prior art cutting nozzles for postmixed oxy-fuel gas torches is their direct exposure to splashback of molten metal from the cut. Splashback metal tends to stick to the discharge end of the nozzle, frequently blocking discharge orifices. When this occurs, the torch must be shut down to permit the nozzle to be cleaned or replaced. This interrupts workflow and increases operating expenses.